1. Field of the Invention
The present invention relates to an image heating apparatus configured to heat a toner image formed on a recording medium and to an image forming apparatus including such an image heating apparatus.
2. Description of the Related Art
Lately, a fixing apparatus is often used in an image forming apparatus employing an electro-photographic unit such as a copier, a printer, a multi-function printer or the like. The fixing apparatus is configured to fix a toner image by heating and pressing the toner image onto a sheet, i.e., a recording medium, when the recording medium passes through a fixing nip portion formed between an image heating member such as a fixing roller and a pressure member such as a pressure roller. The image heating member and the pressure member in the fixing apparatus is merchandized also as a combination of a roller member and a belt member or of belt members, beside the combination of the roller members.
By the way, an image forming apparatus for use in a print-on-demand market in particular is required to speed up in forming images and to be able to accommodate to various kinds of media lately. In speeding up the formation of images, it is preferable to assure a quantity of heat that enables to continuously fix toner images on recording media because a temperature of a fixing nip portion tends to drop significantly when the recording medium (thick sheet) whose basis weight is large passes through the nip portion and it may lead an occurrence of poor fixing.
Then, Japanese Patent Application Laid-open No. 2004-37555 proposes a fixing apparatus (image heating apparatus) configured such that a fixing roller is heated from outside by an external heating member such as an external heating roller brought into contact with the fixing roller as a technology of improving heating performance and to improve productivity in printing thick sheets.
Still further, in a case of dealing with the various types of media, an optimum quantity of heat is different per each medium from aspects of image nature such as a toner offset nature and glossiness of images and conveyance such as wrinkles, waviness of the recording medium, fixing separation between a thin sheet whose basis weight is small and a thick sheet or between a non-coated sheet and a coated sheet. Therefore, Japanese Patent Application Laid-open No. 2006-78555 proposes a technology of controlling a temperature adjustment temperature (target temperature) of the image heating member described above by switching to an optimum temperature corresponding to each type of the recording media to improve a quantity of heat supplied per unit time.
Japanese Patent Application Laid-open No. H9-114286 also proposes a technology of shortening a time required to change the temperature adjustment temperature (target temperature) by cooling the image heating member by blowing air from a fan to the image heating member.
However, productivity in the case where media are mixedly loaded is problematic if the method of switching the temperatures as described above is employed. That is, in a case where a so-called “media consolidated job (consolidated job)” in which different types of media, e.g., thin and thick sheets, are mixed is to be carried out, it takes a time to change the target temperatures of the image heating member, thus generating a stand-by time. The stand-by time is prolonged further in the image forming apparatus trying to improve productivity in printing the thick sheets as described above because the image heating member thereof is constructed such that its thermal capacity is large.